The stimuli that regulate these processes are both mechanical and chemical and include distention of the intestinal wall and the presence of increased [H+], high osmolarity, and nutrients in the intestinal lumen. These stimuli result in a set of changes that represent the intestinal phase of the response to the meal: (1) increased pancreatic secretion, (2) increased gallbladder contraction, (3) relaxation of the sphincter of Oddi, (4) regulation of gastric emptying, (5) inhibition of gastric acid secretion, and (6) interruption of the migrating motor complex (MMC). The goal of this chapter is to discuss how such changes are brought about and how they result ultimately in assimilation of nutrients. Changes in small intestinal function that occur after the meal has passed through will also be addressed.
Gastric Emptying in the Small Intestinal Phase
Immediately after a meal the stomach may contain up to a liter of material that will empty slowly into the small intestine. The rate of gastric emptying is dependent on the macronutrient content of the meal and the amount of solids it contains. Thus solids and liquids of similar nutritional composition will empty at different rates. Liquids empty rapidly, but solids do so only after a lag phase, which means that after a solid meal, there is a period of time during which little or no emptying occurs (Fig. 30.1
Regulation of gastric emptying is achieved by alterations in motility of the proximal part of the stomach (fundus and corpus) and distal part of the stomach (antrum and pylorus) as well as in the duodenum. Motor function in these regions is highly coordinated. Recall that during the esophageal and gastric phase of the meal, the predominant reflex response is receptive relaxation. At the same time, peristaltic movements in the more distal part of the stomach (antrum) mix the gastric contents with gastric secretions. The pyloric sphincter is largely closed. Even if it opens periodically, little emptying will occur, because the proximal portion of the stomach is relaxed and the antral pump (antral contractions) is not very strong. Subsequently, gastric emptying is brought about by an increase in tone (intraluminal pressure) in the proximal portion of the stomach, increased strength of antral contractions (increased strength of the •Fig. 30.1 Ratesofemptyingofdifferentmealsfromadog’sstomach.Asolution(1%glucose)isemptiedfasterthanadigestiblesolid(cubedliver).Notethelagphaseforemptyingofthesolids,whichisrelatedtothetimeneededtoreduceparticlesbelow2mminsize.(AdaptedfromHinderRA,KellyKA.Am J Physiol 1977;233:E335.)100 80 60 40 20 21 3 4 % emptied Hours400 mL 1% glucose 50 g cubed liver Protein Glucose